Treatment of bio-solids is a worldwide environmental problem. As opposed to soil or industrial waste, bio-solids are a direct result of human and animal waste excrement, in short waste of biological origin. For example, bio-solids are produced as the remaining product from a waste water treatment plant after waste water processing is complete and the cleaned water is substantially removed. Bio-sludge is an example of a bio-solid. However, bio-sludge is typically a higher moisture version of bio-solids relative to some of the other common bio-solids. Soil, such as soil being process for soil remediation, is not a bio-solid.
U.S. Pat. No. RE 35251 to van den Broek discloses a sewage sludge treatment system in which gaseous discharge from a pelletizing drier used in the treatment of sewage sludge is partially directed back to a combustion chamber that generates an effluent which is fed to the drier. Volumetric requirements of a gas scrubber and an afterburner are reduced to the volume of gaseous discharge not recycled back to the combustion chamber. A concentrated stream of sewage sludge is mixed with a quantity of dehydrated particulate matter and supplied to a rotary pelletizing drier. Fuel and air undergo a combustion process and are mixed with additional air and part of the gaseous discharge in the combustion chamber which generates a hot gaseous effluent that is directed through the drier. The effluent removes moisture from the mixture of concentrated sludge and dehydrated particulate matter to provide dehydrated particulate sludge and the gaseous discharge. Entrained materials are initially separated from the gaseous discharge by cyclone separators. A gas flow proportioning valve is disposed in a duct system interconnecting the cyclone separators, gas scrubber and combustion chamber for directing a portion of the gaseous discharge back to the combustion chamber.
U.S. Pat. No. 5,195,887 to Peterson et al. discloses a process in which contaminated or uncontaminated soil may be fed to a secondary dryer 28 which is heated with exhaust gas from a primary dryer 22. This cools the hot exhaust gas and, if the soil is contaminated, may serve to drive off moisture and VOCs. If the soil discharged from the secondary dryer 28 is still contaminated it may be fed to a primary dryer 22 which is heated by a burner 20. The exhaust gas from the secondary dryer 28 passes through a conduit 41, exhaust fan 42, venturi 44 and conduit 45 into a particulate control chamber 46. Cooling and reduction of velocity of the gas/fines mixture is achieved as the gas fines mixture proceeds over the surfaces 78 and 80 of airfoils 76 in the particulate control chamber 46 ('887, col. 7, lines 31-33). Gases then pass from the particulate control chamber 46 to a baghouse 52. However, this process treats contaminated soil, rather than bio-sludge, and would be unsuitable for treating bio-sludge.
A principal concern for bio-sludge treatment processes is the need for a high throughput capacity to quickly and efficiently treat the huge quantities of bio-sludge generated by a wastewater treatment plant.